Improved usability of KEditListBox both for the developer and the user:
* Developers can now use setCustomEditor instead of specifying the editor in the constructor. This allows usage in QDesigner.
* Users can properly deselect items to add new ones. The selection is not based on the current item anymore but on the selected one. Thus, users see the selection.
* The lineedit always has focus, making editing easy while up and down keys are mapped to the listview for keyboard selection of items.
* The tab order is now fixed.
* Buttons have icons.

All in all KEditListBox should be really usable now. :)